The present invention is related to a CRT and, more particularly, to its face plate having a light absorbing filter layer having a predetermined absorption peak/peaks.
FIG. 1 shows a partial cross-section of the face plate with a phosphor layer coated of a conventional CRT. There are two sources of visible light coming out of the face panel. One is a light 1 emitted from phosphors when electron beams impinge on them. The other is external ambient light reflected from the face panel. The reflected light has in turn two components depending on where the incident external light is reflected. The first component is that reflected on the surface of the face panel. The other is that passes the whole thickness of the face panel but is reflected off at the phosphor surface. The ambient light reflected from the face plate has a uniform spectrum, degrading contrast of a CRT since the CRT is designed to emit light at only predetermined wavelengths and to display a color image by a selective combination of these predetermined wavelengths.
FIG. 2 shows is a spectral luminescence of P22 phosphor materials commonly used in the art. Blue phosphor ZnS:Ag, green phosphor ZnS:Au,Cu,Al and red phosphor Y2O2S:Eu have their peak wavelengths at 450 nm, 540 nm and 630 nm respectively. Reflected light components 2,3 have relatively higher illumination between these peaks since their spectral distribution is flat across all the visible wavelengths. Spectrum of light emitted from the blue and green phosphor has relatively broad bandwidths and thus some of wavelengths, from 450-550 nm, are emitted from both of the blue and green phosphors. The spectrum of red phosphor has undesirable side bands around 580 nm, at which wavelength the luminous efficiency is high. Therefore selective absorption of light in the wavelengths of 450-550 nm and around 580 nm would greatly improve contrast of a CRT without sacrificing luminescence of phosphors. By the way, because absorption of light around 580 nm makes the body color of a CRT appear bluish, external ambient light around 410 nm is preferably made to be absorbed in order to compensate for the bluish appearance.
Efforts have been made to find a way to selectively absorb light around 580 nm, 500 nm and 410 nm. For instance, U.S. Pat. Nos. 5,200,667, 5,315,209 and 5,218,268 all disclose forming on a surface of the face plate a film containing dye or pigments that selectively absorb light. Alternatively, a plurality of transparent oxide layers having different refraction and thickness were coated on the outer surface of a face plate to take advantage of their light interference for the purpose of reducing ambient light reflection. However, these patents fail to reduce light reflected off at the phosphor layer. So an intermediate layer was proposed, in U.S. Pat. Nos. 4,019,905, 4,132,919 and 5,627,429, to be coated between the inner surface of the face plate and the phosphor layer, absorbing predetermined wavelengths. Further U.S. Pat. Nos. 5,068,568 and 5,179,318 disclose an intermediate layer comprised of layers of high refraction and low refraction alternately.
An objective of the present invention is to minimize the ambient light reflection without using a dye-dispersed layer or a plurality of transparent layers having different refraction.